The present invention relates to dynamoelectric machines and, more particularly, to techniques for cooling end turns of large dynamoelectric machines.
Resistive heating in the field winding of a rotor of a large dynamoelectric machine such as, for example, a large generator or motor, limits the amount of current which can be passed through the field winding to produce a magnetic field. The average current, and the consequent magnetic field strength, must remain below a value resulting in temperatures high enough to degrade the structure of the dynamoelectric machine.
A rotor of a large dynamoelectric machine typically consists of a forging of a magnetic metal having a plurality of longitudinal slots formed therein. Conductor bars are disposed in the slots for carrying energizing current. The ends of the conductor bars are suitably interconnected using conductive end turns to construct a current pattern required to attain the desired distribution of magnetic flux in the rotor.
Both the conductor bars and the end turns, being composed of metal having a finite, non-zero, resistance, give rise to resistive heating.
One passive technique for avoiding excessive temperature in the conductor bars and the end turns includes increasing the rotor size sufficiently to dissipate the expected resistive power dissipation. This solution requires a larger machine with consequent increases in costs of material, installation and foundation.
It is common to employ cooling gas forced through channels in the conductor bars. In some such systems, the conductor bars are formed as a plurality of stacked layers of copper bars. The cooling gas is forced diagonally downward through gas channels aligned in all of the layers, is turned around at the bottom of the stack, and is returned to the surface for recycling. In other systems, cooling gas is forced through at least one longitudinal channel in the conductor bars in thermal contact with the metal.
Some machines using single-layer conductors in the conductor bars and end turns, employ a mitered joint to form the right-angle turn required to permit the end turns to interconnect the required conductor bars. Strength is important in these locations since they are subject to strong thermal cycling. Gussets are brazed in place at the mitered joints to add the requisite strength to the mitered joint.
It is convenient to employ a longitudinal gas channel formed in the surface of the conductor bar to provide a gas channel. The surface is closed by a plate to retain the gas for flow along the entire length of the channel. It is not possible to continue the gas channel through the gusset to the end turn since effectively this would sever the gusset into two pieces and reduce the reinforcement available in the critical area of the mitered joint. Accordingly, it is conventional practice to provide a relatively elaborate and expensive baffling scheme to feed cooling gas to the end turns.